Since vessels formed of thermoplastic resins are superior in shock resistance, and are easily handled, increases of demands are expected from now on. Especially, thermoplastic polyester such as polyethylene terephthalate is superior in transparency in addition to the shock resistance, has a gas barrier property, and has therefore been broadly used in various vessels.
As one example of the thermoplastic resin vessel, there has been a flanged vessel constituted by thermally forming a drawn or non-drawn sheet of a thermoplastic resin.
As a method for manufacturing this type of vessel, for example, there has been a method in which a sheet of softened polyethylene terephthalate is deep-drawn and brought into contact with the interior of a female mold heated at a glass transition point or a higher temperature using a male plug, heat-set, thereafter shrunk back on a male plug, and cooled to manufacture the vessel (Japanese Patent Application Laid-Open No. 58-89319).
According to this forming method, the sheet of polyethylene terephthalate can be drawn to thereby impart transparency, or heat-set to thereby enhance heat resistance.
However, in this manufacturing method, since the bottom part of the vessel is excessively drawn, the bottom part and its vicinity are thinned, and there is a problem that shaping becomes difficult especially in manufacturing a deeply drawn vessel.
Moreover, there is also a problem that a flange portion of the manufactured vessel warps in an upward direction.
An object of the present invention is to provide a method for manufacturing a thermoplastic resin vessel in which thickening of a vessel bottom part and warpage of the flange portion have been improved in consideration of the above-described problem.
To solve the problem, as a result of intensive researches, the present inventors have found that in a step of forming a thermoplastic resin sheet into a vessel by a plug, first the thermoplastic resin sheet is pre-formed by the plug, a pre-formed portion of the thermoplastic resin sheet is next clamped, thereafter the plug is inserted into the sheet to a stroke end to thereby form (draw) a vessel main body, and accordingly the thickening of the vessel bottom part and the warpage of the flange portion are improved, and the present invention has been completed.